1. Field of the Invention
The present invention relates to an analog filter circuit, and more particularly to a frequency tuning loop capable of continuously tuning and compensating the cut-off frequency of a filter.
2. Description of the Prior Art
A filter, which is common and the most important element in a typical signal process system, is configured to eliminate the unnecessary frequency signal for preserving or amplifying the required frequency signal (a filter generally refers to a component which allows/passes a range of frequencies and rejects all other frequencies from an input signal). The filter integrated into the chip is a trend in the development of integrated circuit process. However, many factors can not be properly controlled in integrated circuit process to maintain electrical elements at their perfect properties. Furthermore, the circuit integration may cause the unstable performance of the electrical elements according to the environment and time in use. For example, according to a portable communication product in rapid development, a set of circuit system may be applied in any environments to cause the cut-off frequency of filter aberrant from a preset value. Thus, it is an important key for the signal process of communication system to compensate and keep the cut-off frequency at a preset level.
The cut-off frequency of a filter is proportional to the reciprocal of time constant τ(i.e. f=½πτ) and the time constant τ is the product of equivalent resistance and capacitance (i.e. τ=R×C) of the active RC filter. Therefore, the cut-off frequency may be tuned to a preset value by efficiently controlling the equivalent resistance or equivalent capacitance in an active RC filter. Accordingly aforementioned, a tuning loop is added in the general RC filter for tuning the cut-off frequency back to the preset value by immediately tuning the equivalent resistance or equivalent capacitance in the filter, as soon as the original cut-off frequency of the filter is aberrant. FIG. 1 is a schematic diagram illustrating a known tuning loop. In general, input signal 112 is processed by the RC filter 110 and then output signal 114 outputs from the RC filter 110. As soon as the time constant of the RC filter 110 is aberrant from a preset value (compared through the line 122) and detected by the tuning loop 120, a tuning signal is generated from line 124. Then, the equivalent resistance or equivalent capacitance in the RC filter 110 is tuned for making the cut-off frequency back to the original setting.
FIG. 2A is the typical circuit of a tuning loop with a resistor R22 and a capacitor C22 as the equivalent resistor and equivalent capacitor in the filter. When the constant current sources I22, I24 with a constant current ratio are respectively through the resistor R22 and capacitor C22, a voltage V22 is generated across the resistor R22, while the voltage V24 of the capacitor C22 will increase in accordance with the charging of the capacitor C22 shown in FIG. 2B. When the constant current source begins charging to the capacitor C22, the pulse comparator 220 starts count at the same time; and then stops after reaching a preset time cycle. The preset time cycle is a reference value of time constant. The voltage comparator 210 checks whether voltage V24 is greater than voltage V22 at the moment of stopping counting by the pulse comparator 220. The result acquired by the voltage comparator 210 can be used to decide either increase or decrease the value of capacitor C22 by the tuning loop 230, as well as the charge time of capacitor C22. The tuning loop would repeat the aforementioned process to make the charge time of voltage V22 on the capacitor C22 equal the preset time cycle. Finally, the equivalent capacitance of tuning loop is equal the one of the capacitor C22.
In FIG. 2A, both the resistor R22 and capacitor C22 are grounded at one end respectively. However, it is not the situation in a practical filter circuit. Thus, the equivalent resistance and capacitance in the real filter circuit are different from the ones in FIG. 2A. Therefore, it is necessary to design one having the cut-off frequency of the tuning loop equal the real filter circuit under the accuracy requirement.